Which of these numbers is composite? ${2,\ 31,\ 33,\ 61,\ 67}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 2, 31, 61, and 67 each have only two factors. The factors of 33 are 1, 3, 11, and 33. Thus, 33 is the composite number.